This invention relates generally to the field of manufacturing, and more particularly, to the use of digital models of parts and fixtures, and specifically to a method for analytically modeling the nesting of a part into a fixture for performing a manufacturing procedure.
Machine tools are capable of very precise movements relative to the working surface of the tool. Typical manufacturing operations performed in this manner may include drilling, turning, milling, routing, welding, etc. The accuracy of such a manufacturing operation is limited not by the ability of the machine tool to perform a desired movement, but rather, by the ability of the operator to position the part accurately within the coordinate system of the machine tool. Highly skilled operators using precision measuring instruments are capable of performing machine/part setups for very precise operations. However, the cost of such precision setups is prohibitive for most applications involving high volume operations.
It is known to utilize fixtures to secure a part relative to the working surface of a machine tool during a manufacturing operation. The precision fixture is a mass production solution to the cost of precision setup. Rather than having a skilled machinist precisely position each part to be machined, the use of a precision fixture allows a machinist of lesser ability to rapidly and precisely place a part in a machine tool. In its simplest application, a skilled machinist would set up the machine and fixture, and a less skilled individual would then load the parts and operate the tool to perform the manufacturing operation. Conventional wisdom is that precise parts require very precise fixtures and that fixtures must be made to tolerances that are significantly smaller than their respective part tolerances. Standard practice for inspection or measurement operations is that the gauge tolerance should be only ten percent of the part tolerance. To a large extent, this standard is also applied to fixtures. There are four problem areas commonly associated with precision fixtures. These problems are cost, availability, accuracy, and error documentation. Furthermore, these problems tend to be closely interrelated. The most basic of these problems is cost. In addition to the initial cost of manufacture, there are costs of validation, maintenance, and rework. Contact points on fixtures are subject to wear, and fixture shapes must be verified periodically, with occasional rework being necessary to return them to their specified dimensions. There is also the cost of setting up an incorrectly shaped fixture or adjusting the setup to compensate for part-to-part or lot-to-lot differences.
Availability is related to cost, since all too often, extra fixtures are held in inventory in case they are needed. The manufacture of a precision fixture can be a very time-consuming operation, and in most applications, the demands, of production require that one or more spare fixtures be available at all times. In the aggregate, the cost of this inventory may be significant.
The problem of accuracy can be exacerbated in some applications, for example laser drilling. A small deviation in the shape of a fixture may translate into a very large error in the location of a machined feature. In a laser drilling application, for example, the holes in the surface of a part may be formed at a large angle relative to the surface normal at the drilling point. At angles such as 70 or 80 degrees from normal, even a small error in the location of the surface will result in a large error in the location of the hole.
The documentation problem is generated by the common practice of making manual adjustments to numerically controlled (NC) tool paths or the use of mechanical shims to adjust the location of a part within a fixture. Such practices may remain undocumented, and when the process is moved to a new location or discontinued for a period of time, such undocumented adjustments may be lost, resulting in cost and quality problems when the process is later reinitiated. In cases where fixtures are shimmed or NC programs are altered, the true as-manufactured product definition may be impossible to establish.
Analytical and measuring techniques have been developed to avoid or to minimize such problems. These techniques often involve the modeling of a part and/or a fixture with a Computer Aided Design (CAD) tool, but they usually require manual trial-and-error methods to determine the correct relative positions of the part and fixture models. What is needed is a robust and automatic method for determining the correct position of a part model relative to a fixture model, especially in cases where the part and/or fixture models deviate from their nominal shapes.
With actual hardware, a part may be said to be xe2x80x9cnestedxe2x80x9d into a fixture when there are only tangential surface-to-surface contacts between the part and the fixture, and when there is no way to rotate or to translate the part without causing loss of contact at one of the contact points. Similarly, the analytical nesting of parts involves positioning a part model into a fixture model so that these same two conditions are satisfied: first, there is only tangential surface-to-surface contact between the two models; and second, there is no way to move the model without causing either a loss of contact at one of the contact points or a non-allowable intersection (overlap) between the two models. In rare cases, it may be possible to obtain a non-unique nesting position, in which only the first condition (tangential contact) is satisfied, and it is possible to move the part through some range of motion while maintaining tangential contact. For example, consider a sphere resting on three point supports. In general, fixtures are designed so that corresponding parts have a unique nesting position. In cases where there are non-unique nesting positions, the method described here will find one of the possible nesting positions.
While the nesting of actual parts into actual fixtures is a relatively simple operation for a human being, the nesting of computer models of parts into computer models of fixtures and the recording of the resulting transformation matrix is not a simple task. For example, when the two models start from an overlapped position, existing computer tools define two interfering surfaces as having a zero distance between them, without providing any information regarding the depth of penetration of the interference. This makes it very difficult to determine how to move the part model to achieve the proper tangential contact.
Thus, there is a particular need to develop a method for nesting computer models of parts into computer models of fixtures and for recording the transformation matrix describing the part/fixture movement during the nesting operation. The is a further need to develop a method for nesting a computer model of a part into a computer model of a fixture starting from a position where the part and the fixture surfaces overlap.
Accordingly, A method of nesting a computer model of a part into a computer model of a fixture, the method comprising the steps of generating a model of a part in a computer tool, the part having a part model surface; generating a model of a fixture in the computer tool, the fixture model having a fixture model surface; generating a model of an inset fixture in the computer tool, the inset fixture having an inset fixture model surface that is inset from the fixture model surface by a distance D sufficient to eliminate any interference between the inset fixture model surface and the part model surface; identifying a first minimum normal distance segment between the inset fixture model surface and the part model surface, the first minimum normal distance segment having as endpoints a part surface minimum distance point and an inset fixture surface minimum distance point; constructing a vector extending a distance D in the direction of the first minimum normal distance segment beginning at the inset fixture surface minimum distance point and extending to a fixture contact point on the fixture model surface; identifying a part contact point on the part model surface corresponding to the fixture contact point, the part contact point being a minimum normal distance from the fixture contact point; recording the part contact point as (X1,Y1,Z1) and the fixture contact point as (x1,y1,z1); transforming points of the part model to corresponding points of a transformed part model by applying a transformation matrix that minimizes the distance between the fixture contact point and a transformed part contact point; recording the transformation matrix.